1. Field of the Invention
This invention relates to two wire DC transmitters and more particularly to a means for converting the output thereof from a non-live current signal, such as 10-50 MA (milliamperes), 4-20 MA or the like to a live zero output current such as 0-50 MA, 0-20 MA, etc.
2. Prior Art
In the prior art there are two, three and and four wire DC current transmitters. Typically, in the four wire transmitters power is provided along two wires and a DC current signal representative of a parameter along a second pair of wires. The three wire transmitter is quite similar to a four wire transmitter but operates on three wires as the power supply to the transmitter and the current signal share a common lead. Such three and four wire transmitters are useful and generally function reliably.
More recently two wire DC current transmitters have gained considerably acceptance. Such acceptance is due to the fact that power is provided to the transmitter from an external source along the same two wires through which the two wire transmitter provides a DC current representative of a parameter to an external load. One problem is that the two wire transmitter typically provides a base current elevated above zero (a non-live zero current) representative of a zero or reference level of such parameter. The two wire transmitter consumes the current from zero current to such base current internally such as 0-10 MA or 0-4 MA. While the majority of new uses for transmitters seems to be for two wire transmitters, certain existent facilities and new projects require the live zero output current signal. In the prior art, the choice for such live zero applications has been the three or four wire transmitters or, perhaps, an expensive, often custom engineered conversion of a two wire signal to a live zero output signal. Briefly, a "live zero" can be thought of as a system where any current different from zero indicates a parameter value.